U.S. Pat. No. 4,561,808 discloses a powder feeding system for a thermal spray gun which provides uniform control of powder feed rate. While this device controls the rate of feed through the feed tube, it is also desirable to measure the feed rate from the hopper itself to give an instantaneous indication of either too high or too low a feed rate which would materially affect the quality of a coating when the hopper is used to supply thermal spraying material.
Specifically, in coating processes, powdered materials are fed into a heat source. The rate at which the powders are fed is a critical process parameter. Prior to the present invention, there was no suitable means for determining powder flow rate while a part was being sprayed. Other commercially available devices were either too slow to respond or could not be calibrated in engineering units for all materials and were too costly.
The commercially available devices use weight loss over a fixed time period to produce a measurement of the flow rate of material from a hopper. This however results in jitter in the display of feed rate due to noisy amplifiers and inadequate filtering when the selected time period is too short, or too slow a response to transients when longer time intervals are selected.
The main object of the present invention is to provide a fast response to transients in the feed rate of powdered material from a hopper or other container and to eliminate the disadvantages of prior art techniques.
These and other objects and advantages of the present invention are achieved in accordance with the present invention by the use of adaptive filtering techniques whereby jitter in the output at short time intervals is eliminated through the use of an extremely low noise amplifier stage and a multipole low pass filter.